Document diff.external and mergetool.<tool>.path

There was no documentation for the config variables diff.external
and mergetool.<tool>.path.

Noticed by Sebastian Schuberth.
Signed-off-by: NJohannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -448,6 +448,13 @@ diff.autorefreshindex::
 	affects only `git diff` Porcelain, and not lower level
 	`diff` commands, such as `git diff-files`.
 
+diff.external::
+	If this config variable is set, diff generation is not
+	performed using the internal diff machinery, but using the
+	given command.  Note: if you want to use an external diff
+	program only on a subset of your files, you might want to
+	use gitlink:gitattributes[5] instead.
+
 diff.renameLimit::
 	The number of files to consider when performing the copy/rename
 	detection; equivalent to the git diff option '-l'.
@@ -671,6 +678,10 @@ merge.<driver>.recursive::
 	performing an internal merge between common ancestors.
 	See gitlink:gitattributes[5] for details.
 
+mergetool.<tool>.path::
+	Override the path for the given tool.  This is useful in case
+	your tool is not in the PATH.
+
 pack.window::
 	The size of the window used by gitlink:git-pack-objects[1] when no
 	window size is given on the command line. Defaults to 10.